Polytetrafluoroethylene (PTFE) can be modified while maintaining its inherent non-melt moldability by copolymerizing tetrafluoroethylene (hereinafter referred to as “TFE”) with at least one other fluorine-containing monomer in an amount of not larger than about 2% by weight. Such a copolymer is known as a “modified PTFE” and is distinguished from a melt moldable TFE copolymer. In the art, thus, the term “modified PTFE” is generally intended to denote a TFE polymer modified with a small amount of a comonomer as described above. The particular attention which has been drawn by modified PTFE is due to the fact that this material has physical properties and/or moldability which are not found in “pure” PTFE.
As the modifier, copolymerizable monomers including per(halo)fluoroolefins different from TFE, e.g. tetrafluoroethylene, chlorotrifluoroethylene, perfluoroalkylvinylethers, perfluorodioxoles have been used for manufacturing modified PTFE. The introduction of the modifier to PTFE is known to reduce the crystallinity of the polymer, to suppress the inherent fibrillation property of the PTFE and to improve several other valuable mechanical and processing properties over those of native PTFE.
A useful measure of the decrease of crystallinity in modified PTFE is provided by the Amorphous Index, as described notably in MOYNIHAN, R. E. The Molecular Structure of Perfluorocarbon Polymers. Infrared Studies on Polytetrafluoroethylene. J. Am. Chem. Soc. 1959, vol. 81, p. 1045-1050. The ratio between the intensity of a IR absorption band centred at about 778 cm−1 and the intensity of another IR absorption band centred at about 2367 cm−1 has been shown to suitably and reliably correlates to the fraction of amorphous phase in the modified PTFE materials. In other terms, this IR intensity ratio has been found to be directly proportional to the polymer chain conformational disorder content.
Nevertheless, the introduction of modifying comonomers, in particular perfluoroalkyl(oxy)vinylethers, in the TFE polymer chains significantly contributes to economics (e.g. variable costs) of the modified PTFE itself, because of the use of expensive modifying monomers, whose price largely exceeds that of tetrafluoroethylene.
There's thus a continuous search in this domain for modified PTFE which possess highest possible amorphous fraction with the minimum required amount of comonomer.
Further, there's a continuous need for a process enabling manufacture of said modified PTFE, having optimized consumption of the comonomer to be used with TFE, so as to achieve a target amorphous fraction.